Interaction of Human Chorionic Gonadotropin (hCG) and Asialo-hCG with Recombinant Human Thyrotropin Receptor.

hCG is a putative thyroid stimulator. The present studies were
undertaken to examine its interaction and that of its desialylated
variant asialo-hCG with recombinant human TSH (hTSH) receptor
(hTSHr). To this end, we transfected a human thyroid carcinoma cell
line (HTC) lacking endogenous TSHr with the full-length cDNA of the
hTSHr. Unlike the wild type, the transfected cells, termed HTC-TSHr
cells, were able to bind bovine TSH (bTSH) with high affinity and
increase cAMP production in response to bTSH stimulation. Of the
hCG forms, intact hCG displayed a weak activity to inhibit [125I]
bTSH binding to HTC-TSHr cells, with 100 mg/L (2.6 x 10(-6) mol/L)
producing maximally a 20% inhibition, whereas asialo-hCG achieved
half-maximum binding inhibition at a concentration of 8 mg/L (2.3 x
10(-7) mol/L). The inhibitory constant (Ki) of asialo-hCG for
recombinant hTSHr was calculated from saturation experiments in the
presence of variable doses of bTSH and a fixed concentration of
asialo-hCG to be approximately 8 x 10(-8) mol/L. The interaction of
asialo-hCG with TSHr was further assessed by studies of the direct
binding of the radioactively labeled hormone to both HTC and
HTC-TSHr cells. [125I]Asialo-hCG binding to HTC-TSHr cells was
4.7%, compared to 1.5% in the wild-type cells lacking TSHr and was
displaceable by bTSH (0.1-100 IU/L), indicating specific binding of
the tracer to TSHr. Functionally, hCG (up to 100 mg/L; 2.6 x 10(-6)
mol/L) proved unable to evoke any significant cAMP response over
basal values in HTC-TSHr cells, as did asialo-hCG. Asialo-hCG, but
not hCG, inhibited bTSH-stimulated adenylate cyclase activity in
the cells in a dose-dependent manner. In conclusion, the present
data show that intact hCG binds only weakly to HTC-TSHr cells and
produces no significant cAMP stimulation, which is at variance with
data obtained in FRTL-5 and Chinese hamster ovary-TSHr cells, but
in good accord with previous findings in human thyroid membranes.
Asialo-hCG, on the other hand, strongly binds to recombinant TSHr
and inhibits the cAMP response to bTSH in HTC-TSHr cells,
indicating that the desialylated hCG variant directly interacts
with the receptor and truly is an antagonist of the hTSHr.